[susumuya]

I find it hard to distinguish the two of them.
Are there any ways to distinguish prepositional phrase from idioms?

How can we categorise idioms into various subtypes?
Like, fall in love (with sb.), run a mile, work out, beside onself, dress up...

If 'keep up sth.' is an idiom, is it still a prepostional phrase?
If yes, why some prepositional phrases are idioms and some are not?

Thanks!!!